1. Field of the Invention
This invention relates to a method for developing a positive acting light-sensitive planogrphic printing plate which is sometimes referred to as a presensitized plate (hereafter, a "PS plate"), and, more specifically, to a method for developing a positive acting PS plate containing an o-quinonediazide compound as a light-sensitive material in an automatic processor.
2. Description of the Prior Art
The positive acting PS plate which has been most widely used heretofore comprises an aluminum support having formed thereon a light-sensitive layer of an o-quinonediazide compound. The o-quinonediazide compound is a compound containing a structural unit of the general formula (I) below. It is known that when exposed to actinic light, this compound decomposes to a carboxyl-containing compound represented by the general formula (II) below (see O. Sus, Liebigs Annalen der Chemie, Vol. 556, page 65 (1944)). ##STR1##
Thus, when a positive acting PS plate consisting of an aluminum support and a light-sensitive layer of the o-quinonediazide is exposed to actinic light through a positive transparency, the o-quinonediazide compound present in the exposed area of the light-sensitive layer changes to the corresponding carboxylic acid. Development of the exposed PS plate with an alkaline aqueous solution will, therefore, result in the removal of only the exposed area of the light-sensitive layer, and the surface of the support is revealed. Since the surface of the aluminum support is hydrophilic, the uncovered portion (non-image area) of the support holds water and repels the greasy ink used in planography. The area (image area) in which the light-sensitive layer remains after development is oleophilic and repels water, but holds the greasy ink.
Various alkaline aqueous solutions are known for use as developers for such positive acting PS plates, but an aqueous solution of sodium silicate is most preferred. The aqueous sodium silicate solution has a poor etching action, i.e., the amount which is etched when an aluminum plate is immersed in a sodium silicate solution is less than that in a sodium hydroxide solution having the same pH, on the aluminum support, and, moreover, its developing ability can be adjusted to some extent by varying the ratio of silicon oxide (SiO.sub.2) to sodium oxide (Na.sub.2 O) in the sodium silicate (generally referred to as the SiO.sub.2 /Na.sub.2 O mole ratio) and the concentrations of the solution.
In recent years, the printing industry has widely used automatic processors for PS plates in order to improve and standardize the plate-making process. Those which are in general use are of the developer spraying type schematically shown in FIGS. 1 and 2 of the accompanying drawings. This type of processor includes a conveying passageway through which a PS plate 2 is conveyed in a direction of the arrow by a conveyor means such as feed rolls 1, and a plurality of nozzle pipes 3 above the conveying passageway (and if also desired, below it as shown by the dotted line). As the PS plate 2 is conveyed in the direction of the arrow, a developer 5 fed from a tank 4 is sprayed against the surface of the light-sensitive layer of the plate through the nozzles 3. If required, the surface of the light-sensitive layer is rubbed by brush rollers (not shown) or the like. Thus, the exposed PS plate is processed. The developer is adapted to be passed through a filter 6 to remove insoluble materials, and then recovered and sent to the tank 4. In the automatic processor shown in FIG. 1, the area of contact of the developer to the tank with air is smaller than in the automatic processor shown in FIG. 2, and, therefore, the developer is less deteriorated by the carbon dioxide in the air.
In processing a PS plates in an automatic processor of this type, it is desirable from the standpoint of working efficiency that once a developer has been charged to the developer tank, as many PS plates as possible should be processed without replacing the developer in the tank. In order to change the exhausted developer, the operation of the automatic processor must be suspended for several minutes, and PS plates cannot be developed during this period.
Since the developer is an alkaline aqueous solution and the exposed portion of the light-sensitive layer of the PS plate contains a carboxyl-containing compound as stated hereinabove, the developer will naturally be degraded as the amount of the imagewise exposed positive acting PS plates developed increases. The accumulation of the ingredients of the exposed light-sensitive layer which have dissolved in the developer, the neutralization of the developer with carbon dioxide in the air, and the decrease of the amount of the developer in the developer tank as a result of its being carried away by the processed PS plates, accelerate the deterioration of the developer.
When an aqueous solution of sodium silicate with a relatively high SiO.sub.2 concentration which has a comparatively broad development latitude (i.e., the range of developing conditions with which it will give proper development results) is used as a developer, SiO.sub.2 precipitates in the nozzles of the processor as the number of processed PS plates increases, thus, posing the problem of nozzle clogging. The frequency with which the developer in the tank must be changed and the automatic processor must be washed is, therefore, increased. On the other hand, when an aqueous solution of sodium silicate having a comparative low SiO.sub.2 concentration is used, the buffering ability of the developer decreases, and the developer itself has a low processing capability (i.e., the area of a PS plate which can be developed without any appreciable decrease in sensitivity by a unit of developer is small). In addition, the developer will easily undergo deterioration by carbon dioxide gas present in the air. The frequency of replacing the developer must also be increased.